The present invention relates to measuring fluid flow and in particular to compensating for errors in measuring fluid flow.
There are many applications for measuring the volumetric flow of fluid within a conduit. In particular in clinical and research medicine the measurement or estimation of volumetric blood flow within a blood vessel is desirable. One method of measurement comprises using transit time to estimate volumetric fluid flow. Transducers made from piezo-electric ceramic material, referred to as crystals, transmit and receive sound energy. Typically, these transducers operate at ultrasound frequencies from tens of Kilohertz to tens of Megahertz. Various methods of performing transit time measurements are available. One method of performing transit time measurements involves using sound. Transit time measures differences in time sound takes to transit an upstream and downstream path in a flowing medium. This difference in transit time is proportional to the velocity of the flowing medium. When a conduit is fully illuminated, the difference in time is proportional to the volumetric flow.
Some transit time measurement systems include only one pair of crystals positioned on opposite sides of a conduit, these crystals simultaneously transmit signals through the conduit to be received by the opposite crystal. The signals are time-shifted by the fluid and measurements are calculated based on the time shifted signals. These measurements include time measurements, flow velocity and volumetric fluid flow.
One problem with transit time measurements is measurement errors due to positioning of crystal probes with respect to the flow of fluid. The problem is that the transit time measurements are dependent upon the angle of the ultrasound path with respect to the flow vector. An angle error can be produced due to a shift in positioning of the probe. For some applications the probes are accessible and can be inspected to determine if the probe has shifted from its original positioning with respect to the flow of fluid. As a result angle error can be corrected by repositioning the probe to its original location or determining the current angle of the signal path with respect to the fluid flow and updating measurement calculations based on the new angle. In other applications, where the probes are not accessible such as in vivo applications, determining whether the probes have shifted and repositioning the probes is very difficult. As a result measurement errors can go undetected and produce significant flow measurement errors.
A system of transit time measurements, developed by Transonic Systems, Inc., attempts to reduce the error caused by angle errors by placing a pair of crystals on the same side of a conduit and bouncing ultrasound signals off of a reflector located opposite from the crystals to the opposite crystal. When an error in positioning occurs, the ultrasound signal path angle increases for one segment of the path and the ultrasound signal path angle decreases the same amount for the second segment. This technique can reduce the angle dependency but does not eliminate it, in addition error angle detection for some applications is not available.
In order to improve on these measurement errors some systems have employed two pairs of crystals. Many systems position the crystals so that the crystal pairs transmit signals which form a path designated as an X pattern through the conduit. In one system, two independent measurements for each pair of crystals is received. The measurements are averaged in order to try and compensate for angle errors. The resulting measurement errors, due to improper positioning with respect to the flow vector, may still be significant. In an alternate system, the crystals transmit from one crystal to another and then the same transmission is repeated along the second path. The system then transmits in the opposite direction. The difference in transit time between the two paths is proportional to the volumetric flow. This is a folded geometry configuration. When the angle of the probe changes on the vessel it gets compensated in one direction with one path, and the other direction with the other path. This method has a tendency to reduce angle error measurement but also does not eliminate it. In addition, the current systems do not provide a method for detecting or compensating for measurement errors due to positioning of the crystals with respect to the fluid flow.
The above mentioned problems with flow meters and transit time measurements and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A method and apparatus are described which detect and compensate for angle errors in transit time measurements using at least one ratio of transit time measurements.
In one embodiment, a method for estimating fluid flow in a conduit using a probe with four transducers is provided. The method includes generating two transit time measurements and compensating for an error angle in the transit time measurements using a predetermined compensation factor. The method further includes generating a flow measurement and estimating fluid flow based on the flow measurement.